Old friend gets a stone (4)
I believe the answer is:
'a stone' is the definition.
'old friend' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'friend' becomes 'pal' (pal is a kind of friend).
'gets' is the link.
(Other definitions for opal that I've seen before include "Type of gem", "Milky white gem", "Delicately-coloured gem", "Precious stone", "Semi-transparent gemstone".)